Control of Enzyme Activity

Question 1

Describe enzyme inhibitors

Answer 1

A compound that binds to an enzyme and reduces its activity.

Question 2

What are the two classes of inhibitor and how do they bind?

Answer 2

Irreversible inhibitors
- bind covalently to the enzyme

Reversible inhibitors
- not covalent bound to the enzyme
- can be either competitive or non-competitive (which in turn can be pure or mixed)

Question 3

Describe irreversible inhibitors

Answer 3

• Binds to the enzyme and permanently inactivates it
• Inhibitor reacts with a specific amino acid side chain, usually in the active site, and forms a covalent bond

Question 4

Describe reversible inhibitors and the two different types

Answer 4

• Bind to the enzyme but can subsequently be released (leaving the enzyme in its original condition

Competitive inhibition:
- inhibitor competes directly with the substrate, for the active site
- Mutually exclusive possibilities (one or the other binds)

Non-competitive inhibition:
- Inhibitor binds at a different site than the substrate
- enzyme can bind substrate, inhibitor or both
- in pure non-competitive inhibition, the binding of I has no effect on the binding of S
- Can be used to tune the activity of the enzyme

Question 5

Describe the kinetic parameters of competitive inhibition

Answer 5

• No change in Vmax because infinite [S] outcompetes the inhibitor
• Increases Km because more substrate is needed to get V = Vmax/2

Question 6

Describe the kinetic parameters of non-competitive inhibition (both pure and mixed)

Answer 6

Pure: binding changes the structure of the active site such that S still binds, but transition state stabilisation is no longer optimal
- Vmax decreases, but Km stays the same

Mixed: Km increases and Vmax decreases.

Question 7

Describe why enzymes have multiple modes of recognition

Answer 7

• Need to be able to turn enzymes on and off for finer control
• They use feedback and feedforward regulation to avoid making unecessary metabolic intermediates

Question 8

What are the methods of enzyme regulation?

Answer 8

• Covalent modification
• Allosteric effects
• Proteolytic cleavage
• Turn gene expression on or off
• Degrade the enzyme

Question 9

What are allosteric enzymes?

Answer 9

Enzymes that have an additional binding site for effector molecules other than the active site

Question 10

Briefly describe how glycogen phosphorylase regulates its activity

Answer 10

Feedforward: to increase ATP production
- senses high AMP and therefore low ATP so makes more ATP
- cellular signals

Feedback: to decrease ATP production
- Caffein and purines
- low AMP

Question 11

describe ‘catalytic perfection’

Answer 11

• the upper limit for Kcat/Km is the diffusion controlled limit. ie. the rate at which the enzyme and substrate diffuse together
• this sets an absolute upper limit at ~10^9 s/M
• Enzymes with Kcat/Km above 10^8 s/M are considered to be perfect catalysts